Initial Commit

backend/src/astronomy/time.rs

@@ -0,0 +1,20 @@
+use chrono::{DateTime, Utc};
+
+/// Compute Julian Date from a UTC datetime.
+pub fn julian_date(dt: DateTime<Utc>) -> f64 {
+    let unix_seconds = dt.timestamp() as f64;
+    // J2000.0 epoch is 2000-01-01 12:00:00 UTC = Unix 946728000
+    // JD of Unix epoch 0 = 2440587.5
+    unix_seconds / 86400.0 + 2440587.5
+}
+
+/// Compute Local Sidereal Time in degrees [0, 360).
+/// Uses IAU formula via Julian Date and observer longitude.
+pub fn local_sidereal_time(jd: f64, lon_deg: f64) -> f64 {
+    // Days since J2000.0
+    let d = jd - 2451545.0;
+    // Greenwich Mean Sidereal Time in degrees
+    let gmst_deg = 280.46061837 + 360.98564736629 * d;
+    let lst = (gmst_deg + lon_deg).rem_euclid(360.0);
+    lst
+}